White LEDs which are a kind of white light-emitting semiconductor device configured so as to output white light on the basis of a combination of a gallium nitride-based light-emitting diode (LED) element and a phosphor have recently come to be used also in illumination applications.
In illumination applications, there is a demand for white LEDs having a color temperature of 3,500K or lower (patent document 1). Production of white LEDs which have such a low color temperature and have a high luminance that renders the LEDs usable in illumination became possible due to a success in the development of high-luminance red phosphors. Examples of the high-luminance red phosphors include red phosphors in which Eu2+ is used as an activator and crystals containing an alkaline earth siliconitride, alkaline earth silicate nitride, or alkaline earth silicate are used as a base, such as CaAlSiN3:Eu, which is disclosed in patent document 2, (Sr, Ca)AlSiN3:Eu, which is disclosed in patent document 3, Ca1+xAl1−xSi1+xN3−xOx:Eu, which is disclosed in patent document 4, and (Sr, Ba)3SiO5:Eu, which is disclosed in patent document 5. These red phosphors have a broad emission band having a full width at half maximum exceeding 80 nm and, hence, white LEDs employing these phosphors usually have a high color rendering index (CRI).